Controlling multiple voltage levels for electrostatic printing

ABSTRACT

An electrical arrangement associated with the control and operating circuitry for an electrostatic printing machine of the type employing magnetic brush development for controlling the electrical bias on the developing magnetic brushes for enhancing print quality in situations wherein the background or density of an original to be copied is unsuitable for normal mode printing. Along with normal mode printing, there is provided means for biasing the magnetic brushes for originals having dark background or are of low density.

United States Patent 1 Feldeisen et a1.

[ CONTROLLlNG MULTIPLE VOLTAGE LEVELS FOR ELECTROSTATIC PRINTING [75]inventors: Ronald F. Feldeisen, Henrietta;

Michael R. Stanley, Pittsford, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

' Conn.

[22] Filed: June 11, 1971 [21] Appl. No.: 152,138

[52] US Cl. 118/637, 117/175 [51] Int. Cl .1 G1l3g 13/00 [58] Field ofSearch 118/7, 8, 637, 2;

[56] References Cited UNITED STATES PATENTS 3,219,014 11/1965 Mott eta1. 118/637 3,654,893 4/1972 Piper et a1. 118/2 3,152,012 10/1964Schaffert 118/637 3,185,051 5/1965 Goffe 95/l.7

3,368,894 2/1968 Matkan et a1 96/1 3,328,193 6/1967 Oliphant et a1.118/637 3,037,478 6/1962 Lace 118/637 Primary Examiner-Mervin SteinAssistant ExaminerLeo Millstein [57] ABSTRACT An electrical arrangementassociated with the control and operating circuitry for an electrostaticprinting machine of the type employing magnetic brush development forcontrolling the electrical bias on the developing magnetic brushes forenhancing print quality in situations wherein the background or densityof an original to be copied is unsuitable for normal mode printing.Along with normal mode printing, there is provided means for biasing themagnetic brushes for originals having dark background or are of lowdensity.

4 Claims, 2 Drawing Figures PATEIRITED APR 2 3' 19?";

m 1 OF 2v INVENTORS RONALD F. FELDEISEN MlCHAEL R. STANLEY ATT kA/EYPATENTEDAPRZS I974 3.805739 SHEET 2 UF 2 LINE VOLTAGE a '5 w VOLTAGE I Il SOURCE l f (HIGH) K! 34 32 VOLTAGE I SOURCE U E (NORMAL)| IK2 VOLTAGEI SOURCE I F (LOW) HIGH DENSITY Z? |K 4/ ir j VOLTAGE MACHINE SOURCE i ILOGIC LOW 2K DENSITY 45 VOLTAGE 1 y ApC. coao TRON SOURCE OSCILLATOR 30CONTROLLING MULTIPLE VOLTAGE LEVELS FOR ELECTROSTATIC PRINTING Thisinvention relates to electrostatic printing machines, and in particular,to a control arrangement of voltage supplies for providing developingbias in accordance with the illumination characteristics of an originalto be copied.

Electrostatic printing machines of the type employing an illuminationsystem for illuminating an original generally are encumbered by aninability to present image rays for imaging purposes of sufficient depthor contrast asto produce properly an electrostatic latent image ofusable condition for all possible input originals. For example, anoriginal may have a dark or colored background and when exposed thereresults a la tent image having charged and discharged areas both withhigh charge levels of very close values. When developed, the image willinclude intolerable amounts of background development. On the otherhand, if the original is of the low density type, there usually resultsimage areas almost completely washed out, so to speak. There have beenattempts to offset these problems by varying illumination time. However,these attempts also adversely affect other areas of machine operation orthe rate of production and therefore become inefficient.

Therefore, the principal object of the present invention is to improveelectrostatic printing machines of the type employing magnetic brushdevelopment.

Another object of the present invention is to improve image qualityobtainable from electrostatic printing for originals which in themselveshave very poor irradiance characteristics.

The foregoing objects are attained by an arrangement of electricalcircuitry and control devices which are adapted to be preset fororiginals which are classified as different for controlling energizationof the development bias on the magnetic brushes utilized in the printingmachine. This control of energization is in the form of connecting thebrushes to voltage sources of different voltage levels thereby producingan electric field between the brushes and an electrostatic latent imagein accordance with the need for the particular level so supplied. Thecontrol arrangement may also be associated with other processing devicesin the printing machines in order to control their operation in theenhancement of print quality.

Further objects and advantages will become apparent after reading thefollowing specification in conjunction with the accompanying drawingswherein:

FIG. 1 is a schematic sectional view of an electrostatic reproductionmachine embodying the principles of the invention; and

FIG. 2 is a schematic representation of a control arrangement associatedwith some of the processing devices in the machine.

For a general understanding of the illustrated copier/reproductionmachine, in which the invention may be incorporated, reference is had toFIG. 1 in which the various system components for the machine areschematically illustrated. As in all electrostatic systems such as axerographic machine of the type illustrated, a light image of a documentto be reproduced is projected onto the sensitized surface of axerographic plate to form an electrostatic latent image thereon.Thereafter, the latent image is developed with an oppositely chargeddeveloping material to form a xerographic powder image, corresponding tothe latent image on the plate surface. The powder image is thenelectrostatically transferred to a support surface to which it may befused by a fusing device whereby the powder image is caused permanentlyto adhere to the support surface.

In the illustrated machine, an original D to be copied is placed upon atransparent support platen P fixedly arranged in an illuminationassembly generally indicated by the reference numeral 10, arranged atthe left end of the machine. While upon the platen, an illuminationsystem flashes light rays upon the original thereby producing image rayscorresponding to the information areas on the original. The image raysare projected by means of an optical system for exposing thephotosensitive surface of xerographic plate in the form of a flexiblephotoconductive belt 12 arranged on a belt assembly generally indicatedby the reference numeral 114.

The photoconductive belt ssembly 14 is slidably mounted upon two supportshafts one of which is secured to the frame of the machine and isadapted to drive the belt 12 in the direction of the arrow at a constantrate. During this movement of the belt, the reflected light image of theoriginal D on the platen is flashed upon the photoreceptor surface ofthe belt to produce electrostatic latent images thereon at an exposurestation A.

As the belt surface continues its movement, the electrostatic imagepasses through a developing station B in which there is positioned adeveloper assembly generally indicated by the reference numeral 15, andwhich provides development of the electrostatic image by means ofmultiple magnetic brushes as the same moves through the developmentZone.

The developed electrostatic image is transported by the belt to atransfer station C whereat a sheet of copy paper is moved between atransfer roller and the belt at a speed in synchronism with the movingbelt in order to accomplish transfer of the developed image solely by anelectrical bias on the transfer roller. There is provided at thisstation a sheet transport mechanism generally indicated at 16 adapted totransport sheets of paper from a paper handling mechanism generallyindicated by the reference numeral 17 to the developed image on the beltat the station C.

After the sheet is stripped from the belt 12, it is conveyed into afuser assembly generally indicated by the reference numeral 18 whereinthe developed and transferred xerographic powder image on the sheetmaterial is permanently affixed thereto. After fusing, the finished copyis discharged from the apparatus at a suitable point for collectionexternally of the apparatus.

The belt 12 comprises a photoconductive layer of selenium which is thelight receiving surface and imaging medium for the apparatus, on aconductive backing. The belt is journaled for continuous movement uponthree rollers 20, 21 and 22 located with parallel axes at approximatelythe apex of a triangle. During exposure of the belt 12, the portionthereof being exposed is that part of the belt run between the roller 20and the lower roller 21. The upper roller 22 is rotatably supported on ahollow shaft 23 which is rotatably driven by a suitable motor and drive(not shown) mounted in the machine. Further details regarding thestructure of the belt assembly 14 and its relationship with the machineand support therefor may be found in the copending application Ser. No.102,312 assigned to the same assignee.

As shown in FIG. 1, the electrostatic printing machine employs magneticbrush development, utilizing a plurality of magnetic brushes 25, whichspan across the development zone B. The brushes are mounted for rotationon shafts 26 each of which is electrically connected to a bus bar 27 bysuitable conductive brush elements 28. As is known in electrostaticdevelopment using magnetic brushes, it is desirable, for developmentquality and completeness, that a magnetic brush have associatedtherewith an electric field between it and a latent image beingdeveloped. The combination of magnetic flux lines and electric fieldlines enhances development, provides for solid area coverage and permitscontrol of the uniform application of toner particles to image areas andthe minimization of toner deposition in background areas.

The illustrated electrostatic machine also includes a pre-transfercorona generating device or corotron 30 which, when energized with ana.c. potential at a predetermined level, serves to modify to somedegree, d.c. toner charges on the photoreceptor surface of the belt 12after development and before the transfer of the developed image on thebelt 12 to a sheet of paper at the transfer station C. This applicationof an a.c. current reduces the level of the potential of the image areaso that the potential of the transfer voltage necessary to effect thetransfer of toner particles from the developed image areas to the papermay be lessened. In turn, this lessening of the transfer voltage lessensthe electrostatic forces holding the sheet upon the biased transferroller 31 making strip-off of the sheet an easier operation. For thosemachines which utilize a corotron for transfer purposes, the pretransfercorotron lessens the electrostatic force variability between the sheetand the photoreceptor surface thus making strip-off more reliable for awide range of images.

In FIG. 2 there is shown a control system incorporating electricalcircuitry associated with the machine described above. The bus bar 27 iselectrically connected to a terminal 32, which in turn is connected toan electrical power supply generally indicated in FIG. 1 by the numeral33. Specifically, the bus bar is connected to a voltage source 34 asshown in FIG. 2 which is incorporated in the power supply 33 suitablyconnected to line voltage for the machine. This circuit including thesource 34 is indicative of one provided on electrostatic machinesemploying magnetic brushes for development purposes. This circuitsupplies development bias to the terminal 32 and to each of the magneticbrushes for the general all around use of the machine. Incorporated intothis basic circuit in order to provide for development of originalswhich are either too dark in overall format or of low density, bothconditions being beyond the capabilities of normal electrostaticreproduction, are various control devices for aiding in the reproductionof the originals so conditioned.

As shown in FIG. 2, the general power supply also includes twoadditional voltage sources 35, 36 which are adapted to be connected tothe terminal 32. The electrical connection of either of the sources 34,35 or 36 to the terminal and consequently, the bus bar 27, is selective.As previously indicated, the normal arrangement for general all arounduse of the machine, the source 34 is utilized and is connected to thebus bar by way of two normally closed relay switch contacts 1K2 and 2K1.Therefore, in the normal mode of machine operation, when the machine isin print condition, the magnetic brushes 25 are energized with adeveloping bias supplied by the voltage source 34. For illustrationpurposes, it will be assumed the bias in normal mode operation is 200volts.

The voltage source 35 is electrically connected to the bus bar byclosing the normally open relay switch contact 1K1 and the normallyclosed relay contact 2K1 and the source 36 is electrically connected tothe bus bar by way of the normally open relay switch contact 2K2.Actuation of the contacts 1K2 and 2K1 to open positions and the switches1K1 and 2K2 to closed positions is obtained by means of the two relays1K and 2K electrically connected for energization thereof by the machinelogic 40 and a suitable voltage supply 41 therefor. Associated with therelay 1K for controlling energization thereof is a manually actuableswitch 42 suitably mounted on the control panel for the machine.Similarly, the relay 2K is adapted to be energized upon manual actuationof another switch 43 also suitably mounted on the control panel.

In operation, it will be assumed that an original has a dark backgroundsuch as one in heavy color. In this situation, it is preferable that thedeveloping bias on the magnetic brushes be higher than normal since theoverall charge on the image areas resulting after exposure of this typeof original is higher than normal. This is the resultof the lessening ofthe dissipation of charge during the exposure step of machine processingsince the dark background, like the image areas did not allow sufficientilluminating light rays to reach the photoreceptor belt 12 to dissipatethe previously applied uniform charge in image configuration. With thistype of original the machine operator before commencing actual printing,actuates the switch 42, labeled the High Density switch, for energizingthe relay 1K. This energization actuates the contact 1K1 to a closedcondition and the contact 1K2 to an open position. This results in thenormal mode voltage source 34 being disconnected from the bus bar andthe high voltage source 35 to be connected thereto. For illustrationpurposes, it will be assumed that the source 35 is adapted to'supply 400volts to the bus bar. In utilizing the parameters 200 volts for thesource 34 and 400 volts for the source 35 that for dark backgroundoriginals, a development bias of approximately twice the amount asnormally used can be applied.

Assuming the original is characterized in having low density, it ispreferred the development bias be less than normal since the imagecontrast level remaining on the photoreceptor belt after exposure islower than adequate for normal development. For achieving acceptablecopies from this type of original, the machine operator, beforecommencing the printing, will actuate the switch 43, labeled Low Densityswitch, for connecting the relay 2K to the voltage source 41 therebyenergizing this relay. This energization causes the opening of thecontact 2K1 for disconnecting the magnetic brushes from either of thetwo voltage sources 34, 35 and the closing of the contact 2K2 forconnecting the low voltage source 36 directly to the brushes. Thiscauses the development system to be more sensitive to low contrastimages. For illustration purposes, in relation to the other assignedvoltage values, the voltage supplied by the source 36 may be in theneighborhood of 100 volts.

So that only one voltage source at a time can be utilized, the switches42, 43 may be arranged so that actuation of one to a closed condition,opens the other and when the machine is in normal mode operation, bothswitches 42, 43 assume a non-actuated state.

To enhance development of an electrostatic latent image of an originalof the low density type, the low density control relay 2K is alsoadapted to actuate a normally closed relay switch contact 2K3 to an openposition for effecting the deenergization of the pretransfer corotron30. As shown in FIG. 2, the corotron is connected to an ac. oscillator45 to be energized thereby producing a.c. corona upon the photoreceptorbelt before a developed image is moved into the transfer position C. Aspreviously stated, the pre-transfer corotron is utilized to neutralizeor reduce the charge on the image and background areas in order tolessen the variations due to images necessary for effecting the transferof a developed image to a sheet of paper at the transfer station. Whenthe image and background potentials are nearly equal, as is the casewith low contrast originals, the neutralizing effects of thepre-transfer corotron may further reduce or even eliminate the contrastcharacteristics of the image as presented to the transfer station. Topreclude this from occurring, the low contrast images that have beenenhanced by the low development bias are preserved for transfer byturning off the pre-transfer corotron. For the parameters chosen by wayof illustration, better results have been obtained with the pre-transfercorotron deenergized when the low density switch 43 is actuated.

From the foregoing it will be appreciated that the inventioncontemplates the selective use of three voltage potentials fordevelopment control, depending upon the mode of machine operation; thatis, in the normal mode, for the reproduction of originals of dark orcolored background, or for the reproduction of originals of low densityinformation. Coupled with the latter mode is the choice of utilizing thecontrol of pretransfer conditions. This selective use enables thereproduction of originals not otherwise available in printing machineswhich operate in the conventional normal mode.

What is claimed is: 1. An electrostatic printing machine having aphotoconductive member movable relative to a developing zone, a magneticbrush developing device for develop ing electrostatic latent images onthe member and normal mode circuit means for establishing an electricfield of predetermined electrical potential between the magnetic brushdevice and the latent image during development of the same comprising afirst circuit including a voltage source of a potential higher than saidpredetermined potential,

21 second circuit including a voltage source of a potential lower thansaid predetermined potential, and

control means associated with said circuits for selectively connectingone of said circuits or said normal mode circuit means to the magneticbrush device thereby impressing one of three potentials between themagnetic brush device and the latent image.

2. The machine in claim 1 wherein the normal mode circuit means includesa normally closed switch for normally supplying the magnetic brushdevice with electrical potential.

3. The machine in claim 2 wherein said first circuit includes a normallyopen switch between the voltage source associated therewith and themagnetic brush device and also includes means for opening said normallyclosed switch and for closing said normally open switch therebyeffectively opening the circuit of the normal mode circuit means andclosing said first circuit.

4. An electrostatic printing machine having a photoconductive membermovable relative to a developing zone, means for uniformly charging themember, a magnetic brush developing device for developing electrostaticlatent images on the member, means for transferring a developed imagefrom the member to the support material, and an electrical energizingdevice adapted to neutralize some of the charges on the developed imageprior to transfer thereof comprising,

means for establishing a first electric field between the magnetic brushdevice and a latent image during development thereof,

means for disrupting said first electric field and establishing a secondelectric field in its place and for rendering ineffective theneutralization action of the electrical energizing device.

1. An electrostatic printing machine having a photoconductive membermovable relative to a developing zone, a magnetic brush developingdevice for developing electrostatic latent images on the member andnormal mode circuit means for establishing an electric field ofpredetermined electrical potential between the magnetic brush device andthe latent image during development of the same comprising a firstcircuit including a voltage source of a potential higher than saidpredetermined potential, a second circuit including a voltage source ofa potential lower than said predetermined potential, and control meansassociated with said circuits for selectively connecting one of saidcircuits or said normal mode circuit means to the magnetic brush devicethereby impressing one of three potentials between the magnetic brushdevice and the latent image.
 2. The machine in claim 1 wherein thenormal mode circuit means includes a normally closed switch for normallysupplying the magnetic brush device with electrical potential.
 3. Themachine in claim 2 wherein said first circuit includes a normally openswitch between the voltage source associated therewith and the magneticbrush device and also includes means for opening said normally closedswitch and for closing said normally open switch thereby effectivelyopening the circuit of the normal mode circuit means and closing saidfirst circuit.
 4. An electrostatic printing machine having aphotoconductive member movable relative to a developing zone, means foruniformly charging the member, a magnetic brush developing device fordeveloping electrostatic latent images on the member, means fortransferring a developed image from the member to the support material,and an electrical energizing device adapted to neutralize some of thecharges on the developed image prior to transfer thereof comprising,means for establishing a first electric field between the magnetic brushdevice and a latent image during development thereof, means fordisrupting said first electric field and establishing a second electricfield in its place and for rendering ineffective the neutralizationaction of the electrical energizing device.